The subject innovation relates generally to thyristors and in particular to controlling thyristors to facilitate reducing power loss in a variable speed motor.
Electric motors account for a significant portion of total energy used in the United States and elsewhere. One popular type of motor is the single-phase induction motor, which can be used in blower and fan applications related to home, commercial, and industrial applications, for example. A popular type of single-phase induction motor is a permanent split capacitor (PSC) motor.
An example application can be utilizing a single-phase induction motor in a refrigeration system—typically air conditioning and refrigeration systems discharge heat to the outdoor ambient. To maintain desired (e.g., optimal) refrigerant pressures and system efficiency in varying ambient temperatures, speed of the motor and condenser cooling fans associated therewith are controlled. One technique to control speed of a motor, such as a PSC motor, and associated cooling fan is to employ a thyristor, such as a triac, to vary speed of the motor. This technique is relatively simple and low cost, but does not drive the motor with desired efficiency. For instance, many motors driven at half speed by this technique require half of full speed power, while the mechanical power required to drive the cooling fan is only one-eighth of the full speed requirement. The difference between the input and output power of the motor is dissipated as heat. As such a motor is cooled by the air moved by the fan, motor temperature can increase as fan speed decreases, which can lead to shortened motor life.
Another technique to control motor speed can be to utilize a variable frequency drive (VFD), which can synthesize variable frequency and voltage to facilitate controlling motor speed. However, in many applications, such as certain original equipment manufacturer (OEM) applications, it is not cost effective to use VFDs to control motor speed.
Still other techniques to control motor speed and reduce motor heating involve using two triacs. For example, in one technique, one triac connects an auxiliary circuit to a power line and the other triac modulates power line current to a main winding, where the auxiliary triac is on unless the motor is off. In another technique, two triacs are placed in series, where one triac controls the auxiliary winding and the other triac connects the main winding to the auxiliary winding. This can result in reduction in motor power at half speed depending on the motor—such techniques are not efficient at certain speeds (e.g., lower motor speeds).
It is desirable to efficiently control motor speed to reduce power loss, reduce motor temperature, and increase motor life. It is also desirable to do so in a cost effective manner.